1. Field of the Invention
This invention relates to a pressure sensor including an electrostatic capacitance detection circuit for detecting a scalar quantity of a predetermined external action by utilizing an electrostatic capacity varied by the predetermined external action, and more particularly to an improved pressure sensor for detecting pressure of fluid to generate a signal in proportion to the detected pressure, which is suitable for installation to a gas meter.
2. Discussion of the Related Art
There has been hitherto proposed a pressure sensor including a housing made of metal, a diaphragm supporting a movable electrode with soldering which is fixed by the housing with soldering, a ring spacer made of plastics fixed to the housing by a screw, and an iron plate serving as a stationary electrode which is fixed by a screw opposing to the movable electrode. Due to the construction in which the movable blade is fixed to the diaphragm by soldering, the selection of the material for the diaphragm is restricted and the application of the proposed pressure sensor is restricted. In a production line of the pressure sensor, a special equipment for soldering is necessary, so that the productivity of a mass production for the sensor is deteriorated. Since the sensor has the construction made by the pair of movable and stationary electrodes, the deterioration of the linearity in the sensor by an incidental capacity cannot be avoided. If the linearity is desired to be retained, the compensation by a unique microcomputer must be prepared, which is not suitable for a high-precision application. The sensor is impossible to be corrected about humidity, so that it is not suitable for industrial use.
There has been proposed another pressure sensor having a connector disposed on a diaphragm at its center for pushing a movable electrode, and a lead from the movable electrode. The construction guiding the lead from the movable electrode invites deterioration in its assembling efficiency. Moreover, if negative pressure is applied to the sensor, an excessive force is applied to a center of the diaphragm causing performance change or destruction thereof.
A conventional electrostatic capacity detection circuit includes a detection unit in which its electrostatic capacity is changed by an external action, such as pressure, acceleration, displacement, or the like, to detect the scalar quantity of the external action by measuring an absolute value of an analog electrostatic capacity. Recently a microcomputer is often employed to control a system based on the detected value and to display a digital value for the detected value. An analog signal obtained by measuring the electrostatic capacity is amplified to be converted into a digital signal by an analog-to-digital (A/D) converter for application to the microcomputer, whereby control of the system or digital display is executed.
Such a conventional electrostatic capacity detection circuit invites a bit error in the conversion from the analog signal into the digital signal. Accordingly, an expensive A/D converter having a large number of bits is necessary to decrease the bit error. Moreover, a high accurate amplifier for amplifying the analog signal with temperature compensation is required. Due to the design of measuring the absolute value of the electrostatic capacity, a measurement error is caused by imbalance of the relation between the scalar quantity of the external action and the electrostatic capacity which happens when a dielectric constant changes by circumstances of the detection unit.
As an application of a conventional pressure sensor, there has been proposed a gas meter including the pressure sensor, a seismoscope, and a judging circuit for controlling cut-off of gas by receiving a detected signal from the pressure sensor and the seismoscope, which are mounted on a common printed board to be installed in a meter body of the gas meter while a gas introducing pipe to be connected to the pressure sensor is inserted into a measuring chamber of the meter body in the gas meter. The pressure sensor of a board mounting type needs the gas introducing pipe to be connected with the sensor and the insertion of the pipe into the measuring chamber to be sealed by a sealing means presenting a sealing problem.